Pros: Clear and Liquid sound, fabulous mids, small and portable, insane battery life

Cons: Expensive, No USB Charging

This is one of those times when you just have to have the most expensive toy on the block.

The P51 Mustang is one of those products that sit in the high end luxury category of its product range. It is without a doubt a premiere product, with excellent sound quality as a headphone amp. However, I would stress that the gains this product has at its highway-robbery price point are not commensurate at all compared to a similar product sitting at a much lower price bracket.

While this product has sublimely clean sound with excellent, liquid mids and tight bass, this kind of micro detail improvement is just not worth the $400 or so asking price. This is the kind of product that you just get when you have the money to burn, otherwise I'd recommend a lot of nice products at much lower price points that give the same functionality and comparable audio quality, and oft times more features, such as the GoVibe Petite, which is smaller, has swappable batteries, longer battery life, USB charging and a DAC to round out the package. I simply cannot justify getting a product like this when much cheaper products do pretty much the same job, and I would be wary of those who say it is a much better product than the more affordable ones just like it.

The RSA P51 is, as its name implies, the luxury car of portable headphone amps and its price and differentiation from other products of its ilk reflect it. It's something nobody needs, everyone wants, and when it comes to the bottom line, still does the exact same thing its lower-priced brethren does, albeit with a tad more style and pizazz.

I received a loaner of the new Mustang P-51 portable headphone amp from Ray Samuels Audio. The impossibly small Mustang is quite full featured – lithium Ion battery built in, 3 gain settings. Smaller even that the Tomahawk. Same styling as other RSA amps, which is to say a very attractive anodized metal case. The Mustang, according to Ray, can deliver more than 2 times the amount of current that a Tomahawk can. Pretty impressive.

Headphones used for this review were the Audio Technica AT-ESW9, Kenwood K1000, and the Sennheiser PXC-350.

Treble: A: Amazingly smooth and musical, and yet highly detailed. Maybe not quite as extended as the iQube, but if not, very close. Slightly more extended than the Meier 2Move. The unusual instrumentation on Joanna Newsome’s “Cosmia” was very well rendered.

Midrange: A++: OK, here it comes – the best mids of any portable headphone amp EVER. Yes, this includes the iQube, 2Move, and any of Ray’s other amps. The mids on the Mustang have to be heard to be believed. I was SHOCKED. So pretty, so lush and liquid, and yet so neutral and transparent. Joanna Newsome’s harp and voice on “Cosmia” were so good I truly got shivers and goosebumps. The Mustang’s mids are nothing short of THRILLING.

Bass: A+: Full, tight, tuneful, punchy, deep bass. RSA amps all have great bass, and this one is no exception. No bloat, but lots of meat. Really nice. Just about the perfect combination of fullness, power, and control.

Neutrality: A+: Is it just a touch warm? Probably. Does this make it sound colored? NO WAY. It sounds completely natural – maybe to a level I haven’t quite experienced.

Soundstaging: A: The soundstage was deep, wide, and very well defined. It isn’t quite as good iQube in this regard, but this still will not be seen as a shortcoming.

Transparency: A+: It’s amazingly transparent – it has a jaw-droppingly open, clear, are grain-free sound. It really excels here. Music just naturally flows out of the Mustang, in a liquid, natural way.

WOW. The Mustang is a real achievement in terms of portable headphone amps, in absolute terms, let alone for one that is so small. In fact it is hard for me to accept how such a small little headphone amp can sound so very good. But it sounds EXCELLENT. In my opinion, it is the best sounding RSA portable amp I have reviewed (I have not reviewed the SR71A Blackbird, but all the others).

The Mustang takes its place at the very top of the pyramid in the world of portable headphone amps. It’s terrific, and I cannot recommend it highly enough. For the price, at this level of quality, it’s even a bargain. It was so good, it actually required a bit of re-ranking at the top of my list.

Cons: It's not an SR-71b or Shadow. Not really a con, but output matters.....

I bought this headphone amp after having met Ray Samuels at a Headphone show. He made it absolutely clear how much he valued over-engineering his pieces and how much he prioritized long term reliability. I've bought other Headphone amps since (A FiiO E17 which is also a DAC). It offers really stiff competition and it's probably better value for the money. But I'm not sure if it will hold up like this P-51 has. (I will say, no complaints thus far.) The E-17 charge lasts about fifteen hours and I've never been able to actually run this Samuels Headphone amp down. It just goes and goes.......but I will say that the E17 is a little more muscular in its output, which makes a difference. It deserves its reputation as a great timeless design. I use it with the more efficient headphones in my collection. The E17 mates better with 'phones like the V-Moda M-100, where just a touch of top octave boost makes that headphone much more of an "audiophile" proposition. But the Samuels does a GREAT job with my Sennheisers and my Pro 900. I still have designs on getting a SR-71b or a Shadow. Not sure which to focus on.....so many Headphone products, so little time.